Abstract
\textlessp\textgreaterWe evaluated interindividual variation in traits of warming and hypoxia tolerance in a cohort (\textlessitalic\textgreatern\textless/italic\textgreater = 24) of juvenile pacu \textlessitalic\textgreaterPiaractus mesopotamicus\textless/italic\textgreater, acclimated to 26°C, to investigate whether individuals tolerant to warming were also tolerant to hypoxia and whether individual tolerance depended on body mass or intrinsic traits of aerobic metabolism. Two traits of warming tolerance were measured, the critical thermal maximum (CT_\textrmmax) in a static tank, with loss of equilibrium as the endpoint, and the critical thermal maximum for aerobic swimming (CTS_\textrmmax) in a swim tunnel, with fatigue as the endpoint. Two traits of hypoxia tolerance were derived by static respirometry during progressive hypoxia, the critical saturation for regulation of standard metabolic rate (S_\textrmcrit) and the regulation index (RI). At 39.7 ± 0.4°C (mean ± \textlessitalic\textgreaterSD\textless/italic\textgreater), CT_\textrmmax was significantly higher than CTS_\textrmmax, at 38.0 ± 1.0°C. Both traits had very low coefficient of variation within the cohort (CV, 1.1 and 2.6%, respectively), and CT_\textrmmax was not correlated with any other trait. Individual S_\textrmcrit (18.5 ± 7.2% saturation, CV 38.9%) and RI (76.0 ± 16.1% regulation, CV 45.6%) were correlated (\textlessitalic\textgreaterR\textless/italic\textgreater = 0.686), and both were significantly correlated with CTS_\textrmmax (\textlessitalic\textgreaterR\textless/italic\textgreater = 0.472 and 0.475, respectively). This indicates that individuals tolerant to warming were also tolerant to hypoxia, although two individuals with low CTS_\textrmmax were drivers of the correlations against S_\textrmcrit and RI. The CT_\textrmmax, CTS_\textrmmax, and S_\textrmcrit showed no dependence on body mass, but RI increased with mass (\textlessitalic\textgreaterR\textless/italic\textgreater^\textrm2 = 0.286), indicating that larger individuals were more tolerant to hypoxia. When corrected for body mass, RI was no longer correlated with CTS_\textrmmax, further revealing that the correlations were rather tenuous. Finally, we found no evidence that individual tolerance was dependent on intrinsic traits of aerobic metabolism. The results indicate that sublethal indicators, such as CTS_\textrmmax, S_\textrmcrit, and RI, can be useful in exploring correlations among traits of tolerance to warming and hypoxia in fishes, but more studies on more species with larger sample sizes are required to confirm these results and reveal if there are general patterns.\textless/p\textgreater